Power semiconductor devices are well known to those of ordinary skill in the art and are commonly used for electronic power conversion, regulation, and control. As building blocks of power systems, power semiconductor devices operate in both a switching mode and a linear mode. Power semiconductors satisfy such conflicting requirements as low weight and volume, high circuit-level reliability, fault isolation, and diagnostic capabilities. Power transistors are a type of power semiconductor that is used in a variety of applications in the power range from watts to megawatts. While the majority of applications use power semiconductors in switched mode, other applications require devices to operate in the linear region. Such applications include constant-current capacitor charging and discharging, gradual voltage build up at the load (“soft start”), and switching of inductive loads.
Power semiconductors for switched-mode power conversion have to satisfy such conflicting requirements as high efficiency, high switching frequency, high circuit-level reliability, fault isolation, and diagnostic capabilities.
Composite (“Hybrid”) Switch Configuration
Power semiconductors used for switched-mode electronic power conversion, regulation, and control in the range of tens to hundreds of kilowatts commonly operate at lower frequencies approximately below 50 kHz (for silicon devices), e.g., 10-10 kHz. For insulated gate bipolar transistors (IGBTs), this allows them to take an advantage of their lower conduction loss while maintaining acceptable switching losses. In a composite or “hybrid” switch configuration, an IGBT has a parallel-connected metal oxide semiconductor field effect transistor (MOSFET). FIG. 1 is a graph of a prior art Hybrid Switch Conduction Pattern. As FIG. 1 illustrates, as a “faster” device, the MOSFET is turned on first and conducts during the turn-on interval ΔT1 (FIG. 1). When the MOSFET is fully on, the IGBT turns on as well at the end of the turn-on interval ΔT1 and starts carrying the bulk of current during most of the on-time interval. Near the end of the conduction interval, the IGBT turns off (at zero voltage) while the MOSFET remains in the ON state for the duration of the turn-off interval ΔT2. When the IGBT current expires, the MOSFET turns off with low turn-off loss. Thus, the hybrid configuration combines the best properties of both types of semiconductors and allows power converters to operate at higher frequencies while maintaining high efficiency.
The Conventional Hybrid Switch Approach can have some problems, however. For example, one problem is suboptimum efficiency. In FIG. 1, and in prior art devices, the MOSFET conduction time intervals (ΔT1 and ΔT2) are fixed, because these intervals are designed for the worst case switching times for a given IGBT and cannot accommodate changing current, temperature and device to device variations. As a result, for most cases, the MOSFET conducts longer than necessary degrading the overall efficiency because its ON state loss is significantly higher than that of the IGBT.
Another problem is that the Static Forward and Reverse Biased Safe Operating Areas (FBSOA and RBSOA) Fixed Forward and Reverse Biased Safe Operating Areas can limit flexibility of a device to handle different types of conditions. The FBSOA curves define the maximum drain voltages and currents the device can sustain during its turn on or under forward-biased conditions. The RBSOA curve defines the peak drain current and voltage under inductive load turn off when the transistor drain voltage is clamped to its rated drain to source breakdown voltage BVDSS. For example, FIG. 2A is an exemplary graph of prior art MOSFET forward biased safe operating area (FBSOA) curves, and FIG. 2B is an exemplary graph of prior art MOSFET reverse biased safe operating area (RBSOA) curves. It might be expected that a transistor has to operate within fixed boundaries of the FBSOA and RBSOA under all conditions. However, these curves strictly limit only the maximum drain to source voltage ratings. Otherwise, as opposed to indicating absolute limits for a device, the Safe Operating Area (SOA) curves represent areas of “acceptable” reliability, which are often expressed as Mean Time Between Failures (MTBF). Also, the FBSOA curves normally show data for a single current pulse and several different pulse widths at the case temperature of 25° Celsius (C). Because most applications need continuous operation and higher case temperature, the FBSOA has to be recalculated for every specific case.
As a result, conventional hybrid switches cannot react to changing environmental or circuit conditions such as operating at a higher junction temperature in an emergency with reduced coolant flow or providing higher current to a stalled motor. Obviously, devices can be oversized, but it still does not prevent them from being underused in one mode of operation and overstressed in another.
A further issue is a lack of diagnostics and prognostics. At present, with the exception of some previous designs developed by one of the instant inventors (see, e.g., B. Jacobson, “Integrated Smart Power Switch”, U.S. Pat. No. 7,839,201, November 2010 ('201 patent), as well as B. Jacobson, “Integrated Smart Power Switch, U.S. Pat. No. 8,076,967, November 2011 ('967 patent); the contents of each of these patent applications is hereby incorporated by reference in its entirety) it is not possible to determine if anything is wrong with working power semiconductors—it is only possible to examine failed devices and determine causes of failure. The common prediction method of power transistor reliability relies on the device junction temperature. It is based on theoretical models and does not take into account actual operating conditions. For example, with the exception of the inventor's prior patents, known methods generally do not account for a device failure caused by overstressed die contact to the substrate or faulty mounting to the heat sink. The aforementioned patents of the inventor do, however, address some of these concerns, with safe operating area (SOA) protection, and diagnostics algorithms.
A further issue with prior art devices and methods is lack of real-time calibration and inspection capability. No methods of inspecting and calibrating installed transistors according to their power handling capability exist at this time.
Still another issue with prior art devices and methods is that there often can be suboptimum reliability. The conduction pattern of the hybrid switch (see FIG. 1) is predetermined. That is, the hybrid switch of FIG. 1 has no provisions for adjusting time intervals ΔT1 and ΔT2. The fixed conduction pattern does not allow shifting the full load to a single semiconductor for a short time interval even if the controller detects an impending failure of the other device.